1. Field of the Invention
This invention relates to a stabilization system which maintains a platform in a specified orientation relative to the local vertical, while located on a moving vehicle which is subject to both linear and angular motion. Applications of such a stabilization system include a shipboard system used to maintain a locally horizontal surface to stabilize an antenna pointing at a signal source or one used to support a horizon pointing device. Applications of a sensor suite portion of the stabilization system also include warning systems for land or sea-based vehicles or platforms which alert occupants of a vehicle to prevent accidental rollover of the vehicle.
Until recently, antennas used for ship communication were large (3 to 4' in diameter) and heavy (over 100 Lbs.). Now, with the advent of mobile communications satellites, antenna sizes for land mobile vehicles have been significantly reduced to under 30 lbs. This new situation has opened the opportunity for the use of such antennas on smaller ships, if an economical method of eliminating the effects of ship roll and pitch is available. The availability of a small, low cost stabilized platform would also allow the use of the new, small land-based direct broadcast TV antennas on these smaller ships.
Another area where such a small, low cost stabilized platform would be useful is in the application of radar on small boats, such as fishing boats and pleasure yachts. A stabilized platform for radar would provide much improved clarity under extreme sea conditions.
2. Description of the Related Art
The approach taken by the related art can be described in two broad categories: 1) pointing and stabilization are performed together with one mechanism; 2) pointing and stabilization are separate mechanisms.
This invention addresses the second category and, more specifically, the stabilization mechanism only. Further, this invention addresses active stabilization as opposed to passive stabilization. Typically, passive stabilization systems use weights as pendulums to control their platforms/antennas.
Most related art ignores the fact that the location of the stabilized platform center-of-gravity (c.g.) directly affects the torques required from the control driver motors. Even when this fact is recognized, it is ignored in design considerations, apparently because, for the large antennas considered in the prior art, the additional torque required is small compared to the torque required to drive the antenna. For small, light weight antennas, the increase in torque needed to stabilized the antenna is significant when the c.g. is offset from the axes of rotation. This additional torque increases as a direct function of the distance from the ship's axes of rotation. For small yachts this distance can be easily 25 feet, while for larger ships it can be 100 or more feet.
Stabilization of a platform in a specified orientation to the local vertical requires a control system to have a continuous estimate of the direction of local vertical. Prior art uses several techniques to provide the required information. A description of these techniques is provided in paragraphs (1) through (5) below.
(1) External Reference Data, Typified by Data Provided by the Host Vehicle's Reference System.
This technique is efficient, and adds little complexity. However, the reliance on the vehicle's system prevents stabilization system autonomy and does not solve the stabilization for vehicles, such as small ships, that do not have an applicable reference system.
(2) Inertial Reference Units Including Combinations of Gyroscopes and Accelerometers.
This technique provides local vertical estimates while also maintaining a coordinate reference system for estimating linear and angular motion of the host vehicle. These units require periodic calibration to compensate for drift of components. When only the estimate of the direction of the local vertical is necessary, these units are overly complicated, expensive and heavy.
(3) Passive Approaches Typified by Pendula and/or Various Forms of Inclinometers.
These components are relatively simple and inexpensive. However, they do not measure the direction of the local vertical but rather the direction of the local acceleration vector which will include the effects of any present linear or angular accelerations. The presence of such accelerations in moving vehicles results in unacceptable errors when this approach is employed for the applications being addressed herein. Without compensation for these acceleration effects, this technique is inadequate for the applications being addressed.
(4) Single Axis Acceleration Compensation for Inclinometers.
While appropriate for some applications (e.g. road graders), the present invention will provide stabilization about two axes.
(5) Accelerometer Arrays Providing Essentially the Same Reference System as that Provided by an Inertial Reference Unit.
This technique continuously estimates the angular acceleration vector (three directions) of the base on which the sensors are mounted. The acceleration data are twice integrated to provide a continuous estimate of the angular orientation of the base and, therefore, a continuous estimate of the direction of local vertical. The sensors employed are subject to periodic calibration requirements and are subject to random walk divergence over time introduced in the integration of imprecise or noisy measurements of the angular acceleration. This technique also requires the precision mounting of a minimum of nine accelerometers relative to one another in both linear and angular orientation. To obtain high precision, linear separations must be significant, but mechanical flexure of the mounting surface must be precluded.